Glime, J. M. 2013. Adaptive Strategies: Life Cycles. Chapt. 4-6. In: Glime, J. M. Bryophyte Ecology. Volume 1. Physiological
Ecology Ebook sponsored by Michigan Technological University and the International Association of Bryologists. Last updated
19 February 2013 and available at <www.bryoecol.mtu.edu>.
4-6-1
CHAPTER 4-6
ADAPTIVE STRATEGIES:
LIFE CYCLES
TABLE OF CONTENTS
Life Strategies .....................................................................................................................................................4-6-2
Clonal Growth.....................................................................................................................................................4-6-3
Foraging and Sharing ...................................................................................................................................4-6-4
Implications for Reproduction .....................................................................................................................4-6-5
Density Effects.............................................................................................................................................4-6-5
Tradeoffs ......................................................................................................................................................4-6-6
r & K Strategies...................................................................................................................................................4-6-7
Bet Hedgers..................................................................................................................................................4-6-7
Dedifferentiation Issues ...............................................................................................................................4-6-8
The r Strategist.............................................................................................................................................4-6-8
The K Strategist ...........................................................................................................................................4-6-8
Life Cycle Strategies ...........................................................................................................................................4-6-9
Diaspore Banks ..........................................................................................................................................4-6-10
Tradeoffs ....................................................................................................................................................4-6-10
Life Cycle Strategies based on During (1979, 1992) .................................................................................4-6-11
Fugitives............................................................................................................................................................4-6-11
Fugitives.....................................................................................................................................................4-6-11
Colonists............................................................................................................................................................4-6-11
Colonists (sensu stricto).............................................................................................................................4-6-11
Colonists............................................................................................................................................................4-6-11
Colonists (ephemerals) ..............................................................................................................................4-6-11
Colonists (pioneers) ...................................................................................................................................4-6-11
Shuttles..............................................................................................................................................................4-6-11
Annual Shuttle ...........................................................................................................................................4-6-11
Short-lived Shuttle .....................................................................................................................................4-6-12
Perennial (long-lived) Shuttle ....................................................................................................................4-6-12
Perennial Stayers...............................................................................................................................................4-6-12
Perennial stayers (competitive) ..................................................................................................................4-6-12
Perennial stayers (stress-tolerant) ..............................................................................................................4-6-12
Dominants – bogs..............................................................................................................................................4-6-12
Habitat Studies ..................................................................................................................................................4-6-13
Summary ...........................................................................................................................................................4-6-14
Acknowledgments.............................................................................................................................................4-6-15
Literature Cited .................................................................................................................................................4-6-15
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Chapter 4-6: Adaptive Strategies: Life Cycles
CHAPTER 4-6
ADAPTIVE STRATEGIES:
LIFE CYCLES
Figure 1. Strap-shaped leaves of males and capsules with perichaetial leaves of females of the dioicous moss Diphyscium foliosum.
Photo by Michael Lüth.
Life Strategies
By now it must be obvious to you that to understand
the life strategies of bryophytes, one must first understand
the life cycle (e.g. Figure 1). Frahm and Klaus (2001) state
that bryophytes are able to react quickly to such events as
climatic fluctuations because of their short life cycle and
ease of dispersal by spores. It is likely that when all other
plant life has perished from some Earth catastrophe, it will
be the bryophytes that persist, surviving as spores or other
propagules until conditions return to safety and once again
surrounding the earth due to their ability to travel great
distances as "dust."
By definition, the life cycle includes reproduction.
However, even such a widely used term has ambiguities.
In bryophytes, we shall use this term to refer not only to the
sexual reproduction that results from union of sperm and
egg (ultimately resulting in spores produced by meiosis),
but also to the multitude of asexual (vegetative) means by
which bryophytes are able to make more, physiologically
independent plants (Mishler 1988).
This definition
separates reproduction, which can permit relocation, from
growth, which implies the increase in size of a
physiological individual (Söderström 1994). On the other
hand, growth can ultimately result in reproduction, as is the
case when the plant branches and is physiologically
connected, but later the branches separate and become
physiologically independent. As you can see in Figure 2,
the degree of reproduction by propagules depends on
habitat. Among British habitats, short-lived habitats
(wood, bark, farmland, dung) have the highest degree of
propagular reproduction (that is, by vegetative means)
(Herben 1994). On the other hand, the habitats with the
greatest percent of the species are in the middle of the
range of propagules.
Using the principle that extreme conditions might
provide the best test of the limits of an organism, we learn
that in the maritime Antarctic, bryophytes seem to have
enhanced sexual reproduction (Lewis Smith & Convey
2002). This is contrary to the generally accepted belief that
bryophyte fertility decreases toward the poles. Rather,
43% of the bryophytes (19 species) in Marguerite Bay and
Chapter 4-6: Adaptive Strategies: Life Cycles
47% of those on Alexander Island are known to produce
capsules. But Lewis Smith and Convey attribute this to
favorable microclimatic conditions. Nevertheless, in this
extreme environment, the large majority of mosses with
capsules were short, monoicous, acrocarpous taxa,
suggesting that the predominance of dioicous taxa in more
temperate climates may be possible because the
environment is less stressful. In the more extreme
environments of the Antarctic continent, the numbers of
species producing capsules at similar latitudes (68-72°S)
are much less (33%).
4-6-3
of disadvantaged ramets that might later survive in the face
of adversity; increased precision with the sequestering of
space and dispersal of ramets; ability to monopolize
resources for the benefit of the genotype.
They
furthermore include mobility, but I question whether this is
much of a gain when compared to the alternative of wider
distribution of propagules away from the parent.
One possibility that has barely been explored is the
increase of genetic variability through production of these
haploid genets. We had long assumed that the limited
morphological development of the Bryophyta and
Marchantiophyta reflected a limited genetic diversity, a
case to be expected when the dominant generation is
haploid and asexual reproduction is common. However,
contrary to our expectations, moss populations are
characterized by a high degree of isozyme variation, as
shown for Ceratodon purpureus (Figure 3) (Shaw & Beer
1999). Cultivation of spores from one specimen of
Drepanocladus (Warnstorfia) trichophyllus produced four
distinguishably different morphologies (Sonesson 1966).
Figure 2. The percent of mosses that form spores or gemmae
frequently or commonly in selected habitats of Great Britain.
Asterisks indicate degree of significance (Chi-square test) when
compared to the whole moss flora of Great Britain (* = P<.05,
**=P<.01, ***=P<.001). Redrawn from Herben (1994), based on
data from Smith (1978).
Although life cycle strategies are obviously important,
especially in extreme habitats, life forms and growth forms
may be more important. During and Lloret (1996) found
that within individual sites in Spain, species with the same
life strategy exhibited similar patterns, and that between
locations, growth forms differed more than life cycle
strategies.
Clonal Growth
At the mature end of the gametophytic cycle,
bryophytes can form masses of related individuals, or
clones. Clones can be defined as groups of individual
plants created by fragmentation, viviparous bulbils, or
apomictic seeds (Callaghan et al. 1992), whereas if gene
flow is present the groups of plants are called populations
(Harper 1977). In other words, clones have the same
genetic makeup as the plant from which they were derived.
In addition to these tracheophytic means, bryophytes create
clones through multiple buds on the protonemata. But, as
already discussed, somatic mutations render even these
"clonal" derivations to be variable in genetic makeup.
Callaghan, et al. (1992) attribute to clonal growth the
ability to sequester space and increase fitness of the
populations. Among the benefits are persistence; spread of
development and reproduction over time and
environmental conditions; risk-spreading between ramets
(individual members of clone) of the same type, thus
increasing chances for survival of the genotype; protection
Figure 3. Color and leaf shapes of Ceratodon purpureus.
Top: Green, broad leaf, hydrated form, Middle: Green,
lanceolate leaf, hydrated form. Lower: Reddish dry form with
capsules. Photos by Michael Lüth.
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Chapter 4-6: Adaptive Strategies: Life Cycles
Velde and coworkers (2001) addressed this question of
genetics of clonal relationships in Polytrichastrum
formosum. They determined that identical genotypes
between spatially separated shoots were almost never
present, whereas identical genotypes among genets
(branching of gametophytes resulting from clonal growth
of rhizomes) was extensive. However, this view of the
genet has some problems. Scrosati (2002) pointed out that
somatic mutations are predictably common, giving rise to
genetic mosaics within any connected genet. To deal with
this lack of genetic homogeneity, Scrosati suggested that
genet should be defined as a "free-living individual that
develops from one original zygote, parthenogenetic
gamete, or spore and that produces ramets vegetatively
during growth." Nevertheless, it appears that even in
adjacent populations of spore-producing plants, genetic
variation is minor. In another example, populations of
Funaria hygrometrica (Figure 4) growing in close
proximity on contaminated tailings of a copper mine
displayed very low levels of genetic variability, but had
extensive morphological plasticity (Shaw & Bartow 1992).
Figure 4. Crowded Funaria hygrometrica with its abundant
capsules. Photo by Michael Lüth.
During and van Tooren (1987) attempted to explain
this paradox of genetic diversity in vegetatively
reproducing taxa. Referring to earlier views of bryophytes
as evolutionary failures, they contended that bryophytes in
fact have high genetic variability, yet maintain their
populations almost entirely by asexual means. They
suggest that even though sexual reproduction, when it
occurs, results in huge numbers of spores, establishment
from spores in the field seems to be very difficult. Rather,
they suggest that "remarkably rapid fine-scale dynamics" of
many bryophyte populations may account for their ability
to maintain a high degree of genetic variability.
Itouga and coworkers (1999) provide data on genetic
variability in the liverwort Conocephalum japonicum.
They consider both the species and populations to have low
genetic diversity values of Hes (species genetic diversity) =
0.008 and mean Hep (population genetic diversity) =
0.008±0.003. Between populations diversity was likewise
low with Gst (coefficient of genetic differentiation) =
0.062. They used this low diversity as an indication that
reproduction by gemmae predominated over sexual
reproduction by spores.
Velde and coworkers (2001) considered this strategy
of producing clonal genets to be one that provided
increased longevity for the genet that accompanies
increased reproductive capacity.
Nevertheless, they
showed that male reproductive success in Polytrichastrum
formosum is determined primarily on spatial distance from
females. In fact, these populations achieved their success
primarily through sexual reproduction, facilitated by the
ability of sperm to disperse to distances measured in meters
rather than mm or cm.
Foraging and Sharing
The reproductive advantages of ramets may be
enhanced by other advantages found more recently, at least
in tracheophytes. In seed plants, the ability to relocate
photosynthate from plant parts in the light to shaded parts
has been demonstrated (Kemball et al. 1992), while other
plants are able to translocate resources through rhizomes
and roots (Landa et al. 1992). This permits the ramets to
take advantage of flashes of sunlight called sunflecks, and
horizontal growth that permits such advantages has been
termed foraging (Bates 1998). Ramets furthermore may
have seasonal advantages as different parts become
exposed to light at different times of the year. Even
nutrient and moisture advantages may accrue if part of the
plant receives sunlight while another part extends into
moister or more nutrient-rich soil. Even in simpler plants
like lycopods (Diphasiastrum flabelliforme), Lau and
Young (1988) demonstrated that ramets that had been
severed from their connecting ramets experienced 50%
more mortality than unsevered ramets. Those ramets
connected to shaded ramets were able to maintain higher
water potentials, giving them the ability to take advantage
of the sun in one ramet while maintaining high water
potential through that supplied by another ramet.
In bryophytes, as in tracheophytes, we can expect
advantages to the clonal habit. Living where their parents
have lived increases the probability that the habitat is
suitable, thus reducing wastage of propagules. A greater
area of soil and atmospheric water is contacted by a clone,
in some cases permitting a greater nutrient capture and the
opportunity to provide needed water and nutrients to the
growing tip. However, the ability to transport hormones,
nutrients, and photosynthate is known for so few examples
of bryophytes that we cannot generalize these benefits. In
some tracheophytes, leaves on different parts of the plant
and within the clone differ in morphology, permitting
different environmental conditions to favor them. Such
differentiation may be possible on rhizomatous taxa such as
Climacium, and some leafy liverworts exhibit different leaf
morphologies on the same branch (e.g. Lophocolea
heterophylla, Figure 5), but no systematic investigation has
explored this as a possible clonal advantage.
Figure 5.
Heteromorphic leaves of Lophocolea
heterophylla. Compare the two leaves indicated by arrows.
Photo by Janice Glime.
Chapter 4-6: Adaptive Strategies: Life Cycles
If indeed clonal transport such as that demonstrated in
tracheophytes is possible in most bryophytes, nutrients
could move internally from favorably placed ramets to
those in less favorable positions in a patchy environment,
benefitting the bryophytes in a competitive environment
(Bates 1998). Bergamini and Peintinger (2002) likewise
compared the bryophytes to tracheophytes, suggesting that
their overall morphological responses to the differences in
light levels approximated that of tracheophytes with stolons
– a "spacer and branching strategy." But does this ability
to share with less favorably placed ramets only work for
bryophytes with internal conduction?
Eckstein and
Karlsson (1999) tested this hypothesis by comparing the
movement of nitrogen in Polytrichum commune, with
well-developed internal conduction, with that of
Hylocomium splendens, with predominantly external
conduction.
Indeed, the labelled nitrogen pool in
Hylocomium splendens moved from older segments to
younger segments.
In Polytrichum commune, it
disappeared from younger segments in autumn, presumably
going to underground storage organs. Both of these
examples support the hypothesis that ramets can provide
sources of translocatable substances from one part of the
clone to another, but we have few studies to permit us to
assess the extent of this phenomenon among bryophytes,
nor does this explicitly demonstrate the transfer from one
ramet to another less favorably positioned. And could
gametophytes such as those in Figure 6 transfer substances
from one gametophore to another through the protonema?
Figure 6.
Circular growth pattern of gametophores
developing from a single spore of Funaria hygrometrica. Photo
by Janice Glime.
There need be no internal conduction to foster other
types of advantages, however. For example, Sphagnum
magellanicum is able to keep its neighbors moist through
its efficient external conduction, and cushion mosses like
Leucobryum (Figure 7) conserve moisture by growing in
dense clones.
Implications for Reproduction
Perhaps there is a division of labor that provides a
reproductive advantage among ramets of a clone that is
independent of type of translocation. Stark et al. (2001)
found that in the desert moss Syntrichia caninervis more
mature ramets with larger size were more likely to
reproduce than the smaller ramets, suggesting a division of
labor that permitted smaller plants to conserve energy until
they achieved a greater size. While this may be simply a
4-6-5
function of age, it would permit the colony to have multiple
reproductive strategies simultaneously, with larger ones
reproducing sexually and smaller ones using only
fragments or vegetative propagules.
Figure 7.
Janice Glime.
Cushion of Leucobryum glaucum.
Photo by
Many of the modes of reproduction of bryophytes
result in clonal growth. Rarely does one see just a single
bryophyte stem. Rather, clumps, cushions, tufts, mats, any
number of growth forms, suggest that these are all siblings
of an original single parent. In fact, even if only a single
spore lands on the rock or soil, many plants arise, at least in
mosses. The spore produces a protonema that branches,
and in the case of the filamentous protonemata, the
numerous branches can give rise to numerous upright
gametophores. Knoop (1984) identifies two types of
gametophore origin: Funaria type (Figure 6), developing
gametophores on the caulonema only in a circular fashion
around the spore; Polytrichum type, developing few
gametophores near the germinated spore or even from the
spore cell itself (Sood & Chopra 1973, Nehlsen 1979).
Both result in several to many gametophores.
In Sphagnum a single spore produces a small thalloid
protonema that gives rise to only one gametophore, thus
resulting in populations when more than one spore
germinates, and making one uncertain in any given clump
of Sphagnum whether the clump is a clone derived from
apical branching or a population derived from separate
spores. However, if one considers that the branching of the
capitulum contributes to a major portion of the mat
growth, then, again, clonal behavior is at work.
Furthermore, spores are likely to land on their own parents
or siblings or cousins of the parent, and thus not be far
removed from clonal relatedness.
Even gemmae can form circular arrangements of
gametophores, as reported by Chopra and Rawat (1977) for
Bryum, or other arrangements of numerous gametophores,
as in Physcomitrium sphaericum (Figure 8; Yoshida &
Yamamoto 1982). Since these have arisen from one parent,
they likewise produce clones. In Bryum bicolor, numerous
tubers and gemmae are produced early in the growth of the
gametophore, permitting it to build up a large clone (Joenje
& During 1977).
Density Effects
Colony density has varying effects on moss success.
In ectohydric mosses, it is more likely that density will
favor success and increase growth (During 1990; Økland &
Økland 1996). But in Sphagnum (Clymo 1970) and
Rhytidiadelphus triquetrus (Bates 1988) density is
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Chapter 4-6: Adaptive Strategies: Life Cycles
detrimental to both branching and growth. Shoot mortality
can increase, as in Polytrichaceae (Watson 1979), or
decrease, as in Tetraphis pellucida (Kimmerer 1991), with
density. Almost nothing is known of density effects on
liverworts; Laaka-Lindberg (1999) showed that density had
no effect on gemma production.
Figure 8. Clone of Physcomitrium sphaericum. Photo by
Michael Lüth.
On the other hand, Kimmerer (1991), found that
density was an important factor in asexual vs sexual
expression in the acrocarpous Tetraphis pellucida (Figure
9). Gemmae were common in low-density colonies and the
sex ratio was female-biased. High-density colonies, on the
other hand, were more likely to have sexual reproduction
and a greater proportion of males. She pointed out the
advantage of this plastic strategy in unstable environments
such as the rotting stump habitat of Tetraphis pellucida,
permitting the plants to expand by gemmae when the
colony was not dense.
Figure 9. Tetraphis pellucida. Upper: Uncrowded plants
with gemmae on tips. Lower: Dense patch of plants with
sporophytes. Photos by Michael Lüth.
Tradeoffs
Traditional life history theory holds that "maximizing
reproductive value at each age is equivalent to maximizing
fitness" (DeRidder & Dhondt 1992).
However, in
bryophytes, as in many species of seed plants, there is a
negative correlation between sexual reproduction and
asexual reproduction (Caswell 1985). On the other hand, in
the clonal insectivorous flowering plant Drosera
intermedia, DeRidder (1990) found only limited evidence
of a tradeoff between the two types of reproduction.
DeRidder and Dhondt (1992) suggest that traditional theory
may apply to the clonal D. intermedia, whereas in many
clonal species, it is an inappropriate theory because it was
based on organisms (vertebrates) with only one mode of
reproduction.
Ramets of one taxon, all from the same spore,
seemingly competing for space and resources, seems like a
maladaptive thing to do. However, the old safety in
numbers adage may apply here. Multiple stems are less
likely to dry out than a single plant. The colony can
acquire a cushion shape as the middle members grow better
due to moisture held by their neighbors. The edge
members are slowed because if they too grow like the
middle members, they are left with no protection from
drought on the outer side. Hence, the adventurous stem
that grows a bit taller is soon stopped by lack of moisture,
and those on the edge are slowed the most because they
lack a similar tall plant on the outside to protect them. For
tracheophytes, Price and Hutchings (1992) also consider
design constraints that limit vascular connections between
some ramets, a consequence that should not be a problem
for the ectohydric bryophytes. Could this, however, reduce
the advantages for endohydric bryophytes, i.e. those
relying significantly on internal conduction?
Perhaps one of the greatest benefits to ramets from a
single spore of monoicous bryophyte taxa is availability of
the opposite sex. Since in many cases, the male and female
gametangia don't mature at exactly the same time on the
same individual, gametangia of a neighbor are more likely
to be receptive than other gametangia on the same plant.
Such an opportunity is not so important to the
tracheophytes because of their dependence on external
pollinators that can readily visit nearby clumps of a
different clone.
Nevertheless, for the bryophytes,
concomitant with the advantages of having nearby sexual
partners are the tradeoffs in disadvantages of marrying your
twin, in particular the loss of genetic diversity.
Like the tracheophytes, bryophytes must pay a price
for the clonal habit. The advantage of being able to
respond rapidly to environmental change is unlikely for the
slow-growing bryophytes. Reduced recruitment from
spores will make the clonal bryophytes vulnerable to
permanent changes in the environment, and the
connectedness makes the entire clone vulnerable to
disturbance (cf. tracheophytes, Callaghan et al. 1992). The
longevity of the clump in habitats like the Arctic make the
probability of frost heave damage an eventuality in some
habitats. Mosses seem less likely than tracheophytes to
reap benefits from having only part of the clone disturbed.
If part of a cushion is removed, the entire cushion is likely
to dry out, whereas an intact cushion is able to keep its
interior moist under most circumstances. In most cases,
spread by spores in less clonal species may be a better
Chapter 4-6: Adaptive Strategies: Life Cycles
4-6-7
strategy, particularly for those that require fresh soil in
open, disturbed areas. Thus, as their habitat changes, they
have the means to move on to other suitable areas.
The trade-offs and benefits of clonal growth,
contributing to increased bryophyte density, are hard to
assess. The overwhelming presence of clonal growth
suggests that it has its advantages for bryophytes, perhaps
almost completely in the greater moisture retention.
r & K Strategies
Life cycles are basic to the survival of a species.
Those that are annuals must usually survive the winter as
spores. Those that are perennial must have other ways to
survive the cold of winter. Still others may live where it is
a dry season, not winter, that must be reckoned with.
These differences in seasonal stresses are generally met by
differences in life strategies.
Ramensky (1938) described three types of outcomes to
the differences in life strategies as violents (aggressive
species), patients (tolerant species), and explerents (noncompetitive species that fill the spaces between others).
Rabotnov (1975) added pioneers (species able to colonize
substrata that are not yet suitable for other species).
Meanwhile, MacArthur and Wilson (1967) introduced the
concept of r and K selection as the extremes of a life cycle
strategy continuum, and the western world seemingly
ignored Ramensky and Rabotnov, generally only using the
term "pioneer" among these. Although r and K strategies
were largely described to fit animal concepts, many of the
ideas can be applied as well to plants.
The r-selected species are characterized by a rapid
growth rate, early reproduction, numerous, small offspring
(spores or seeds in plants), and a high resource uptake; the
r can be compared to the r (intrinsic growth rate) in the
logistic population model. The r strategist is likely to be
a short-stayer, adapted to disturbed or ruderal (field &
wasteland) habitats where it is necessary to arrive quickly
and mature before the habitat changes. By contrast, the Kselected species is characterized by slow growth rate, late
reproduction, few, large offspring, and efficient use of
resources; the K strategist optimizes for a high population
density at the environment's carrying capacity (cf. the
logistic model). The K strategist is likely to grow where
the habitat is more stable, and it can be a long-stayer,
eventually reaching considerable size (or cover). The K
strategist is more likely to depend on asexual reproduction
such as rhizomes and perennial habit whereas the r
strategist is more likely to rely on seeds or spores and an
annual habit with good dispersal.
Thus K strategists tend to be competitors; r strategists
tend to be opportunists but not competitors. An r strategist
is the more likely one to succeed on unstable, disturbed
environments (sometimes a pioneer, sometimes an
explerent), whereas the K strategist is the more likely one
to succeed in more stable and predictable habitats (the
patient or tolerant species and sometimes the violents or
aggressive species).
Grime (1977) considered the r strategist to be ruderal
(of field or wasteland) species that took advantage of
disturbed habitats (Figure 10). The K strategists he
considered to be the stress tolerators that were able to
survive dry or cold periods as whole plants, the perennial
stayers. Between those two he placed the competitors.
Figure 10. Frequency of ruderal (R), competitive (C), and
stress-tolerant (S) species along an r-K continuum. Redrawn from
Grime (1977).
Bet Hedgers
But between these two extremes are lots of
possibilities for having some of the characteristics of each.
Few species can meet all the criteria of either, and tradeoffs abound to permit the organisms to meet the demands
of their particular habitats. Plants that seem to have both
good sexual reproduction and a means of vegetative
reproduction are bet hedgers. Like the people to whom we
refer as bet hedgers, these plants are "unwilling" to put "all
their eggs in one basket." They use two strategies
simultaneously so that they do not lose entirely. The price
they pay is that they likewise never win entirely – at the
ends of the spectrum, there is either an r strategist or a K
strategist that is better adapted to the circumstances.
Production of gemmae among sexually capable species
is one example of bet hedging. In the dry interior of North
America, Syrrhopodon texanus (Figure 11) exhibits
seasonal production of gemmae, arising in August (33% of
specimens) and climaxing in September (50% of
specimens) (Reese 1984). In this species, rarity of males
makes this bet hedging a desirable strategy, although
sporophyte-bearing females invariably occur when males
are present.
Figure 11. Gametophytes of Syrrhopodon texanus. Arrow
in lower picture indicates gemmae at leaf tip. Photo by Janice
Glime.
4-6-8
Chapter 4-6: Adaptive Strategies: Life Cycles
Dedifferentiation Issues
It is somewhat difficult to consider bryophyte r and K
strategists in the same way as that of seed plants. These
two strategies rely heavily on three characteristics of the
plant life cycle: arrival and persistence, establishment and
growth to maturity in a developing community, and time
taken for the species to reach critical life stages (During
1992). Bryophytes are problematic because they do not
follow a consistent pathway from spore (propagule) 
juvenile  immature individual  reproductive individual.
They can revert, growing from a fragment into an adult, or
growing from a broken tissue of a fragment into a
protonema  juvenile  immature individual 
reproductive individual.
Bryophytes are able to
dedifferentiate – return a cell to its embryonic
(undifferentiated) state
Compared to most tracheophytes, most bryophytes
would appear to be r strategists, utilizing many small
progeny (spores) that travel great distances and having
short life cycles, permitting them to move on to new
locations easily. This may help to account for the
widespread distribution of many bryophytes relative to that
of seed plants. However, when compared to phanerogams,
most bryophytes do not meet the requirement for rapid
growth.
Since most species will fall between the two extremes
of r and K, the first thing one must realize when trying to
determine the r or K status of a species is that ascribing r or
K must be done in the context of comparison. Thus, within
bryophytes, both ends of the continuum exist, while most
species have a mix of characters.
Although bryophytes typically produce large numbers
of small spores, many taxa also can increase in numbers by
stolons, rhizomes, and branching, qualifying them as K
strategists, or long-term stayers. For example, Hedenäs and
co-workers (1989) found that the invading moss
Orthodontium lineare in Sweden had a high spore output,
but that colonies had a clumped pattern that indicated
strong neighborhood effects that permitted spread within a
locality. Thus, within the bryophytes, as in tracheophytes,
species can be divided into r and K strategists, but they are
unlikely to meet all criteria of either, and many trade-offs
exist (Stearns 1989). Instead, it appears that many of them
are bet-hedgers, being prepared to take advantage of
whatever comes along instead of being prepared with a
single strategy.
To succeed, they must balance their energy
expenditure between sexual reproduction and vegetative
growth in a way that best permits them to survive. These
strategies must of course be coordinated with their entire
physiology and the methods by which each developmental
stage is signalled.
The r Strategist
Like typical r strategists, bryophyte r strategists rely
heavily on massive numbers, typically 50,000 per capsule,
of small spores (10-15 µm) to get to a new location
(Schofield 1985). For example, Funaria hygrometrica can
arrive quickly on disturbed sites such as soil charred by fire
or agricultural land. But should this be true in predictably
disturbed sites such as flood plains? In flood plains one
finds members of Archidiidae (Figure 12), a subclass of
large-spored mosses, with spores usually 50-150 µm, large
enough to be seen without a lens (Schofield 1985). Here it
would appear to be advantageous to stay put by producing
large, long-lived spores (Söderström 1994). It is likely that
this stay-put strategy is available to many mosses and
liverworts through spore longevity in soil banks.
Figure 12. A floodplain moss, Archidium alternifolium.
Photo by Michael Lüth.
The K Strategist
Our understanding of perennial stayers (K strategy) is
limited by our ability to determine the age of an individual.
To age a moss or liverwort is somewhat difficult because
among the perennial ones, the bottoms typically die as the
tops continue to grow. However, many mosses carry their
own age markers (Hagerup 1935), as described in more
detail in another chapter, much as trees can be aged by
terminal bud scars while they are young. Polytrichum
males can be aged by counting the number of splash cups
along the stem, because a new year of growth will come
from the cup in the following spring. Ulychna (1963)
found Polytrichum commune with a mean age of 3-5
years, but dead parts in the hummocks ranged 15-17 years.
Brunkman (1936) found Hylocomium splendens (Figure
13) up to 30 years old by counting the successive sets of
branches that form like stair steps, each from a point near
the apex of the old, but it is unlikely that the oldest parts
were still live and functioning. Because most bryophytes
do not require their lower parts to keep the upper parts of
the plant alive, they could theoretically grow indefinitely in
a location due to the growth of the tips. Such a
phenomenon is approached in Sphagnum, which will
continue to grow as long as the habitat remains suitable.
Figure 13.
Hylocomium splendens showing stairstep
branching used for aging the moss. Photo by Janice Glime.
Chapter 4-6: Adaptive Strategies: Life Cycles
Spores, however, are not the only stage in which r and
K strategies might be applied. One could also expect that
there would exist a trade-off between numbers of male and
female gametangia. Just as some trees, such as maples
(Acer) can adjust the number of male and female flowers
based on tree crowding, one might look for regulation of
numbers of male and female gametangia. In their studies
of tropical bryophytes, Cavalcanti Pôrto and Moto de
Oliveira (Moto de Oliveira & Cavalcanti Pôrto 2001;
Cavalcanti Pôrto & Moto de Oliveira 2002) found that
development of gametangia was responsive to rainfall. In
the moss Sematophyllum subpinnatum, the number of
antheridia per perigonium was 8-20 while the number of
archegonia per perichaetium was 3-26.
For
Octoblepharum albidum (Figure 14) mean number of
antheridia per perigonium was 13.4 and of archegonia per
perichaetium 6.7. Could moisture regime change these
ratios?
Figure 14.
Lüth.
Octoblepharum albidum.
Photo by Michael
Just how do the r and K strategies of bryophyte
gametangia line up? Fuselier and McLetchie (2004)
considered this problem in Marchantia inflexa. They found
that females had a greater growth rate, but males had more
asexual reproduction. Males were also more likely to be
present in a high light regime (55% shade), where they
began sexual development earlier; males in low light
produced no sexual structures (McLetchie et al. 2002).
Fuselier and McLetchie (2004) postulated that eventually,
the greater female growth rate would result in a population
of all females as they overgrew males. However, under a
disturbance regime, more males would be successful. They
found a female bias in sex expression, with many
genetically male plants failing to express sexual traits.
The r and K strategies are at best a continuum.
Individual species often do not meet the criteria
completely. Evolution is imperfect and time is required to
drive it toward perfection.
Furthermore, the model
predictions work only if the environment perfectly matches
with the set of bryophyte characters predicted. In the
Antarctic, extreme conditions would seem to test this r and
K continuum to its limits. And there the imperfections of
these predictions are evident. The disturbed nature of this
volcanic habitat favors r-selected taxa that must arrive from
considerable distances (Convey & Smith 1993). However,
the difficulty of spreading during the short, cold growing
season favors certain short-lived taxa with large spores.
Five of the species that are widespread in the Antarctic
have large numbers of small spores and are most likely
long-distance colonists. Even the longer-lived taxa seem to
4-6-9
defy the r & K model predictions, having a large
investment in sexual reproduction.
Life Cycle Strategies
To combat all the insults of the environment that might
be encountered in a global array of habitats and climates, a
variety of strategies exist among both plants and animals.
For bryophytes, the predominant controlling factor is
available moisture, but we must consider that temperature
is also a major contributor to the timing of life cycle events.
As we consider the life cycle strategies of bryophytes,
we must keep in mind that they potentially expose all of
their alleles to expression and selection through a
considerable portion of their lives – as 1n gametophytes.
All the variety in strategies discussed above come into play
in permitting these tiny organisms to occupy the widest
array of conditions of any group of plants. For the greatest
number of species to survive across the greatest number of
habitats, some have adapted to be opportunists, constantly
moving from place to place, while at the other extreme are
perennial stayers, finding a suitable place and remaining
there for a long time. But because an individual bryophyte
must stay in one place, it must have a life cycle that permits
it to survive the onslaught of environmental fluctuations
during the entire time it develops from protonema to leafy
plant to fertilization to sporophyte to dispersal of spores.
The environment thus provides the major selection
pressure on the life cycle strategies. Recognizing the
instability of the environment, Stearns (1976) classified the
environment into three main types (examples are mine):
1. having long cyclic fluctuations, with a period much
longer than that of the generation time of the
organism (e.g. fires)
2. having short cyclic fluctuations, with a period that is
as long as or shorter than the generation time of the
organism (e.g. seasons)
a. cycle highly predictable
b. start of cycle unpredictable
c. start of cycle predictable, but conditions of
growing season unknown
d. start of cycle predictable, but conditions only
partly known
3. having random fluctuations, i.e. not predictable (e.g.
flash floods)
To survive in a fluctuating environment, the life cycle
must prepare the bryophyte for the fluctuations. This
means that at times it is advantageous to "run for your life"
to other locations (produce spores), whereas under other,
more favorable conditions it is best to sit still and keep your
family together (reproduce vegetatively).
During (1979) has examined in detail the life cycle
strategies of bryophytes in dealing with environmental
conditions. In finding that most tracheophyte life cycle
strategy systems either did not apply or were incomplete
for the bryophytes, he devised a system of six strategies.
He considered that bryophytes utilize three major tradeoffs: few large spores vs. many small spores, survival of
stressful season as spores (avoidance) vs survival as a
gametophyte (tolerance), and life span that is negatively
correlated with reproductive effort (for tolerants only)
(During 1992). In addition, there is a usually tradeoff
between sexual and asexual reproduction (Schofield 1981,
During 1992).
These considerations resulted in his
4-6-10
Chapter 4-6: Adaptive Strategies: Life Cycles
organization of strategies based on life span, spore number
and size, and reproductive effort (Table 1 and figures from
During 1992; table slightly modified):
Table 1. Spore and life span characteristics of the various
life cycle strategies for bryophytes as defined by During (1979).
Potential
life
span (yrs)
<1
Few
Many
Numerous
very light
<20 µm
Fugitives
Colonists
Ephemeral
Colonists
Pioneers
Perennial stayers
Competitive
Stress-tolerant
Spores
Few
large
>20 µm
Annual shuttle
Repro
effort
High
Variable
Short-lived shuttle
Long-lived shuttle
Dominants
Low
This system has attributes that work as well for higher
plants, and Frey and Hensen (1995) have proposed a
modified system based on this one to be used for all plants.
(Now how often do you see those tracheophyte folks
copying a bryophyte idea?! Kudos to During!) They have
expanded upon the original six strategies proposed by
During to include nine: annual shuttle species, fugitives,
kryptophytes, short-lived shuttle species, colonists,
perennial colonists, perennial shuttle species, perennial
stayers, and perennial stayers with diaspore years.
Fugitives (Figure 15), colonists (Figure 16), annual
shuttle species (Figure 17), and short-lived shuttle
species (Figure 18) are r strategists and all succeed in
disturbed environments. The fugitive strategy is relatively
rare, with Funaria hygrometrica being one of the few
examples (During 1992). That many species require
disturbance and therefore are relatively rare in any specific
locality is usually overlooked in trying to conserve rare
taxa. The very disturbance they need to persist is often
prevented in an effort to maintain them! Noble and Slatyer
(1979) attribute success following disturbance to plant
strategies related to three factors: method of arrival
(fugitives, colonists, annual shuttle species) or persistence
at disturbed site (short-lived shuttle species); ability to
become established and reach maturity in disturbed site;
time needed to reach critical life cycle stage. These criteria
are not intended to include those of taxa adapted to
continuously disturbed or catastrophically disturbed
habitats, but rather to those recurring events such as fire,
flood, or treefall. The perennial bryophytes are K
strategists (Figure 19, Figure 20) of stable habitats.
During (1992) added the category of dominant to
accommodate taxa with large spores and long life
expectancy, such as some Sphagnum species. It is a rare
combination among bryophytes, whereas it is relatively
common among trees. Other categories will surely be
added as we gain understanding of tropical ecology and the
adaptive strategies of bryophytes there (During 1992). One
such category could develop based on neoteny, where
juvenile characters are retained in adults, a condition that
occurs among some species of ephemeral habitats such as
living on leaves in the tropics (During 1992). In some taxa,
such as Buxbaumia, neoteny permits the species to avoid
some life cycle stages, in this case the leafy gametophyte!
La Farge-England (1996) has suggested the category of
protonema mosses to encompass these few taxa (see
chapter on life forms and growth forms). Others, such as
Dicranum and Fissidens species, have dwarf males that
develop on leaves of female plants, facilitating the transfer
of sperm to the egg, a kind of male neoteny. (See chapters
on sexuality and on the development chapter on
gametogenesis for further discussion of dwarf males.)
Diaspore Banks
Disturbed habitats, whether the product of predictable
natural phenomena or unpredictable events such as human
intervention or volcanic eruptions, benefit from the bank of
spores and asexual diaspores (any structures that become
detached from parent plant and give rise to new
individuals) stored in the soil out of reach of sun and
sometimes even water. Major disturbances can bring these
propagules to the surface where they can break dormancy
and become established. We need only look at a recently
disturbed bank in a forest, sloping deforested hillside, or
crumbling streambank to recognize the importance of
bryophytes in colonizing and often maintaining the surface
integrity. Yet, as Ross-Davis and Frego (2004) pointed
out, while these regeneration processes "may be critical to
conservation of severely disturbed communities..., they are
poorly understood." In an attempt to quantify this
importance they sampled two grids in managed Acadian
forests of New Brunswick, Canada. They identified 51
taxa in the aerial diaspore rain and buried diaspore banks.
Of these, 36 represented species in the existing community
of the Acadian forest. The composition of aerial diaspores
was more similar to the existing community than to that of
buried ones.
Tradeoffs
For bryophytes, the system of success strategies is
complicated by the ability to reproduce from fragments,
and in many cases the production of asexual propagules on
the protonemata as well as on the leafy plant, leading
During to his 1992 revision. One must keep in mind that
bryophytes may be among the best dispersers in the world.
Therefore, large spore size, as opposed to small ones with
worldwide dispersal potential, may be a tradeoff of great
magnitude. While many of these small spores will not
survive the long distance travel due to UV radiation and
other atmospheric hazards (see dispersal chapter later),
many will survive significant local travel, with a few
travelling for hundreds of kilometers.
Once the spores arrive, different attributes become
important. The spore must have sufficient energy to
survive until favorable conditions arise, and it must get the
new protonema off to a good start with enough energy to
survive in some very harsh environments. This has
resulted in a correlation of spore germination patterns with
habitat (Nehira 1987). Epiphytic and saxicolous species of
both mosses and liverworts tend to have endosporic
germination (i.e., early development of several mitotic
divisions within the spore wall; Figure 21), permitting them
to be multicellular when they emerge from the protection of
the spore. This would suggest that these species carry
sufficient nutrients with them to supply their initial
developmental nutrient needs. On nutrient-poor, xeric
(dry) substrates such as rock and bark, internal
development could insure protection during early, critical
stages of development. However, most mosses have
exosporic germination (first mitotic division occurs outside
spore after rupture of spore wall).
Chapter 4-6: Adaptive Strategies: Life Cycles
4-6-11
Life Cycle Strategies
based on During (1979, 1992)
Fugitives
Fugitives – species that live in unpredictable
environments
example: Funaria hygrometrica
short life span; ephemeral or annual
high sexual reproductive effort; large percent of plant
devoted to spore production
low age of first reproduction (first year)
spores small (<20 µm), very persistent and long-lived
no asexual reproduction
innovations absent
open turfs
rare in phanerogams (mustards?) and bryophytes; found
among bacteria, algae, fungi
Figure 15. Fugitive strategy. From During (1979).
Colonists
Colonists (sensu stricto) – species that live
where habitat start is unpredictable, but lasts several
years; secondary succession
bryophyte examples: Bryum bicolor, Bryum argenteum,
Ceratodon, Marchantia
short life span; (annual-) pauciennial-pluriennial
sporophyte late, somewhat rare in many; first sexual
reproduction at least after 1 and usually 2-3 years
high reproductive effort
spores < 20 µm, persistent
innovations present
asexual in early life; first asexual reproduction in a few
months
usually short turf
old field species like Solidago
Figure 16. Colonist (sensu stricto) life cycle strategy. From
During (1979).
Colonists
Colonists (ephemerals) – gap-dependent
species
bryophyte example: Bryum erythrocarpum
short life span; (annual-) pauciennial-pluriennial
first sexual reproduction in a few months
sexual reproduction rare
spores < 20 µm, persistent, numerous
high asexual reproductive effort by subterranean tubers on
rhizoids
river flood plains, low areas submerged in spring,
cultivated fields
usually short turf
Colonists (pioneers) – species that live where
habitat start is unpredictable and habitat lasts at least
several years after disturbance; make habitat suitable
for perennial stayers (Rabotnov 1975)
bryophyte examples: Grimmia, Schistidium
long life span
slow growth
perennial
high reproductive effort
first sexual reproduction in a few years???
sexual reproduction low
asexual reproduction high
spores < 20 µm, persistent
river flood plains, low areas submerged in spring,
cultivated fields
usually short turf
Shuttles
Annual Shuttle – species that require small
disturbances that last 1-2 years; survive severe stress
periods
bryophyte examples:
Ephemerum, Physcomitrium,
Fossombronia
short life span; (ephemeral-) annual-pauciennial
sexual reproduction effort high and frequent
age of first reproduction < 1 year
spores large, 25-50 (-200) µm
survive by spores
capsules often immersed (short or no setae) (Longton 1988)
specialized asexual reproductive structures absent
innovations rare
open turf or thalloid mat
agricultural weeds, hoof prints, steep stream banks, dung
disturbed habitat species like Brassica
Figure 17. Annual shuttle life cycle strategy. From During
(1979).
4-6-12
Chapter 4-6: Adaptive Strategies: Life Cycles
Short-lived Shuttle – species that don't avoid
periods of severe stress; habitat lasts 2-3 years
bryophyte examples: Hennediella heimii, Splachnum,
Tetraplodon
life span several years, pauci-pleuriennial
sexual reproductive effort high; sporophytes more or less
frequent
overall reproductive effort medium
ages of first reproduction 2-3 years
spores large, 25-50 (-100) µm
asexual reproduction rare
innovations present
short turf or thalloid mat
Perennial Stayers
Perennial stayers (competitive) – forest floor
bryophyte examples: Brachythecium rutabulum
long life span
perennials
rapid growth
sexual and asexual reproduction low or nearly absent
age of first reproduction several years
spores <20 µm
spore longevity variable
wefts, dendroids, mats, large cushions
Perennial stayers (stress-tolerant) – fens,
bogs, desert
bryophyte examples: Sphagnum, Syntrichia ruralis
long life span; perennials
slow growth
sexual and asexual reproduction low or nearly absent
age of first reproduction several years
spores <20 µm
spore longevity variable
growth form plasticity
in deserts include acrocarpous taxa with long setae
tracheophytes include ericaceous shrubs
Figure 18. Short-lived shuttle life cycle strategy. From
During (1979).
Perennial (Long-lived) Shuttle – species that
require stable environments, such as epiphytes,
where end of habitat is predictable
bryophyte examples: Orthotrichum, Marchantiales
long life span; pluriennial, perennial
sexual reproduction effort moderate (During 1979) or low
(During 1992)
age of first sexual reproduction high (>5yrs)
spores large (25-200 µm)
spore life span short
asexual reproduction effort moderate
innovations present
age of first asexual reproduction variable
cushion, rough mat, smooth mat, tuft
tracheophytes include bromeliads, Betula, Populus
Figure 19. Perennial long-lived shuttle life cycle strategy.
From During (1979).
Figure 20.
During (1979).
Perennial stayer life cycle strategy.
Dominants – bogs
bryophyte example: some Sphagnum
long life span; perennial
sexual reproduction effort low
spores large (>20 µm)
asexual reproduction effort low
turf
From
Chapter 4-6: Adaptive Strategies: Life Cycles
Figure 21. Endosporic development (arrow) in spores of the
hornwort Dendroceros tubercularis. Photo by Karen Renzaglia.
But spores are not the only way to travel. Fragments
and propagules can carry the species to a new location,
although the generally much larger size would usually limit
distance considerably. Moss balls (see chapter on life
form) along lake shores and on glaciers and snow banks
serve as means of dispersing large units, including multiple
plants. Landslides, rock movement in streams, trampling,
and vehicle tires can carry fragments for some distance.
For those producing asexual propagules, sexual
reproduction and asexual propagules are usually not
produced at the same time. Thus, investment in specialized
asexual structures is indeed a trade-off. Taxa with annual
life cycles, surviving unfavorable conditions as spores,
rarely produce such specialized structures, investing their
energy instead in the production of spores (During 1992).
We know little about the energy costs of producing
spores and other propagules, and in particular know
nothing of the effect of spore production on mortality
(During 1992).
There is evidence, however, that
development of sporophytes slows the growth of the
gametophytic plant in Scorpidium scorpioides (A. M.
Kooijman & H. J. During, unpubl. data) and Plagiothecium
undulatum (Figure 22; Hofman 1991), as well as in
Dicranum polysetum mentioned earlier (Bisang & Ehrlén
2002). This tradeoff may be a general rule, as discussed in
the chapter on sporophyte development.
Figure 22. Dicranum polysetum exhibiting its multiple setae
per stem. Photo by Janice Glime.
4-6-13
Some characteristics of the life strategies may be
interrelated. For example, Hedderson (1995) found that in
the Pottiales the probability of producing capsules
decreased with increased life expectancy and was
negatively associated with asexual propagules.
As
discussed in the chapter on asexual propagules (brood
bodies), these compete for energy with the production of
capsules and generally do not occur simultaneously. It
therefore follows that dioicous taxa in this group have more
asexual propagules, corresponding with their lower
likelihood of having sexual reproduction.
Unlike
sporophytes, asexual propagules were positively associated
with life expectancy. On the other hand, size accounts for
only a small, but statistically significant, proportion of the
variation in life history traits in the Funariales,
Polytrichales, and Pottiales (Hedderson & Longton 1996).
Rather, characteristics related to water relationships were
most important, accounting for 40-50% of the variation. It
is interesting that the ability to take in and retain water
coincides with monoicous taxa that are short-lived and
produce few large spores, whereas those at the opposite end
of the endo-ectohydric gradient have opposite characters.
Spore number and spore size are strongly related to family,
with most of the variation occurring among genera.
Variation among species is moderate. Hedderson and
Longton suggested the possibility of coevolution of water
relations and life history in these orders.
Longton (1997) used the concept of life history
strategies to predict character relationships. Colonists,
fugitives, and shuttle species exhibit an earlier age for first
reproduction as the longevity decreases. These strategies
are accompanied by greater monoicy and reproductive
effort (Longton 1997, 1998). Such species tend to have
more plastic phenotypes and experience greater success at
establishment by spores. Dioicous moss colonists, on the
other hand, are more likely to produce asexual propagules,
whereas such propagules are widespread among liverworts.
Habitat Studies
Occasionally a habitat study will describe the growth
forms or life forms that dominate there. But quantitative
studies to describe these are rare. However, a few
examples from tropical habitats can serve to provide an
understanding of their usefulness in giving a mental picture
of the bryophyte cover in places we have never visited.
In the Colombian cloud forest, epiphytes are
abundant due to the high moisture availability from the
clouds and the infrequency of desiccation events. This type
of climate supports growths of tall turfs and smooth mats as
predominant growth forms on the trees (van Leerdam et al.
1990). On the other hand, the life strategies of bryophytes
on trees on the eastern Andean slopes of northern Peru
reflect the drier habitat. Colonists form short turfs of
acrocarpous mosses, primarily in secondary forests
suffering disturbance. In the lowland and submontane
forests, perennial shuttle species and perennial stayers
exercise low sexual reproductive effort and take advantage
of the high humidity to accomplish high vegetative
reproduction through both propagules and clonal growth
(Kürschner & Parolly 1998a).
Macromitrium and
Phyllogonium fulgens (Figure 23) have dwarf males
resulting from small male spores compared to large female
spores. (Dwarf males are discussed more thoroughly in the
4-6-14
Chapter 4-6: Adaptive Strategies: Life Cycles
chapters on sexuality and gametogenesis.) Leptodontium
viticulosoides exhibits functional heterospory in which
small spores are dispersed long distances and large ones
only short distances. On the other hand, at high elevations
near timberline, the perennial shuttle and perennial stayer
species instead exercise a high sexual reproduction and
produce numerous sporophytes.
Similar altitudinal
differences occur in Southeast Asia and Central Africa.
rapidly to maturity following rain, and produce large spores
in capsules that typically lack stalks and remain submersed
among the perichaetial leaves; often these capsules lack
peristomes and opercula and may be dispersed as whole
capsules (see chapter on development of sporophytes). The
perennial shuttle species are mostly thallose liverworts such
as Riccia that curl up and become dormant or survive as
large spores. Fugitives may arrive, but generally are gone
after 1-2 years, travelling to new sites as small spores.
Figure 24. "Fog-stripping" by thin leaves of Campylopus
holomitrius in the mist from geothermal vents at Karapiti, New
Zealand. Photo by Janice Glime.
Summary
Figure 23. Pendant Phyllogonium fulgens in Japan. Photo
by Janice Glime.
Bryophytes of the tropical lowlands have a very
different character from these montane epiphytes,
providing them with maximum water conservation in this
much drier habitat. The mat life form encompasses species
with water lobules, water sacs, and rhizoid discs
(Kürschner & Parolly 1998b). This life form gives way to
fans, wefts, dendroids, and pendants in the more humid
montane belt. These forms serve as collectors to condense
water vapor from the frequent fog and mist (fog-stripping;
Figure 24). Deeply fissured or ciliate leaves and rill-like
arrangement provide the fine wire-like surfaces needed for
this water capture. The tropical oreal and subandean belt
contrasts with this foggy area by having strongly
contrasting diurnal conditions and supporting short-turf,
tall-turf, and tail life forms with central strands, rhizoids,
and rill-like leaf arrangements.
Bryophytes of arid habitats are typically small and
may include acrocarpous perennial stayers with small
spores and long setae that aid in dispersal (Longton 1988).
Annual shuttle species here are primarily ephemerals that
avoid desiccation by going dormant as spores, develop
Bryophyte life strategies must be closely attuned to
the water regime of their environment.
They
accomplish this fine tuning by using spores, fragments,
and specialized asexual propagules during times when
conditions are not suitable for the gametophyte.
Furthermore, they attune their times of sexual
reproduction to meet the availability of water.
Secondary to the water schedule is the advent of
disturbance for which some bryophytes are especially
adapted (opportunists).
Bryophytes, especially mosses, are clonal
organisms.
All bryophytes are able to spread
vegetatively through fragments and propagules.
Perennial mosses also spread by branching
(ramets/genets). Mosses, additionally, produce many
upright gametophytes from the protonema developed
from a single spore. Clones have the advantage of
maintaining moisture, but have the disadvantage of
being genetically identical. Bryophytes that grow
horizontally have been considered foragers that are
able to take advantage of a patchy environment to
obtain nutrients, light from sunflecks, and even water
in different parts of the plant. They are able, at least in
some taxa, to transport these nutrients or the
photosynthate to other parts of the plant. Sexual
reproduction is favored when clones and clumps
provide both sexes, and even in monoicous taxa the
differences in maturation times among members of the
clone become an advantage.
Density can work for and against bryophytes. At
low densities, water loss is greater and sexual
reproduction is less successful, favoring spread by
spores at high densities. However, in some mosses,
Chapter 4-6: Adaptive Strategies: Life Cycles
such as Polytrichum, shoot mortality can increase with
density, but in other taxa it can decrease.
There is a tradeoff between sexual reproduction
and asexual reproduction, including branching and
growth, as these events compete for energy.
Compared to tracheophytes, bryophytes are r
strategists, but within the bryophytes there is an entire
range from r strategist to K strategist. The r
strategists rely on large numbers of small spores and a
short life cycle (opportunists). K strategists rely on
their clonal, perennial growth (perennial stayers) and
often have only limited sexual reproduction or are
strictly vegetative. But most bryophytes lie somewhere
on the bet hedger line, producing spores sexually, but
using fragments and asexual propagules during seasons
when energy is not needed for sexual reproduction or
spore production.
Because of their ability to
dedifferentiate, bryophytes often spread by fragments
of ordinary tissue.
Availability of water is the most important
determinant of life cycle strategy.
Endosporic
development is more common on low water, low
nutrient substrates like rock and bark. Disturbance is
actually required for some species.
Bryophytes utilize three major tradeoffs: few large
spores vs. many small spores, survival of stressful
season as spores (avoidance) vs survival as a
gametophyte (tolerance), and life span that is
negatively correlated with reproductive effort.
Diaspore banks permit bryophytes to survive
untenable periods of time in a dormant state and begin
growth when suitable conditions return. Endosporic
development permits some bryophytes to get a head
start in particularly short-lived periods of adequate
moisture, such as deserts, floodplains, and vertical
substrates.
Acknowledgments
Jean Faubert caught some serious inconsistencies in
the r-K selection text and made valuable suggestions to
improve this subchapter.
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